<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        main{
            width: 800px;
            margin: 100px auto;
            text-align: center;
        }
    </style>
</head>
<body>
    <main>
        <h4>长度单位换算</h4>
        <form action="">
            <input type="text" placeholder="输入数值：">
            <select name="" id="num1">
                <optgroup label="全部">
                    <option value="100000">千米</option>
                    <option value="100">米</option>
                    <option value="10">分米</option>
                    <option value="1">厘米</option>
                </optgroup>
            </select>
            <select name="" id="num2">
                <optgroup label="全部">
                    <option value="100000">千米</option>
                    <option value="100">米</option>
                    <option value=10">分米</option>
                    <option value="1">厘米</option>
                </optgroup>
            </select>
        </form>
        <div id="result"></div>
    </main>
</body>
<script>
   
</script>
</html>